1. Field of the Invention
The present invention relates to a biodegradable thermoplastic resin composition having cellulose derivatives and a surface-treated natural fiber, and more particularly, to a biodegradable thermoplastic resin composition having cellulose derivatives and a surface-treated natural fiber, in which a density of the natural fiber is increased by subjecting the surface of the natural fiber to an alkali and/or sizing treatment, a mechanical strength of the biodegradable thermoplastic resin composition is improved by increasing the interfacial adhesion between the cellulose derivatives and the natural fiber, and the production cost is reduced using the natural fiber as a reinforcer.
2. Discussion of Related Art
While plastics have brought convenience and wealth to humans, the excessive dependence on the plastics is causing the earth to suffer from a flood of plastic wastes. Fiber-reinforced plastics or polymer composite materials introduced to further improve the characteristics of the plastic are widely used in the field of various applications such as the shipping, automobile, leisure, defense, and aerospace industries. However, since the fiber-reinforced plastics or polymer composite materials are not eco-friendly, they cause environmental pollution. For about a decade, bio composite materials using eco-friendly and durable natural materials have attracted worldwide attention, and thus their steady research and development have been conducted. Such biodegradable plastics can be degraded within 90 to 180 days after being exposed to the air, and thus need not be burned. Therefore, they are very effective in reducing carbon dioxide. Biomass-based materials have taken the spotlight in place of conventional petroleum-based materials as the next generation growth engine for reducing an environmental load such as emission of greenhouse gases and complying with increased international environmental regulations.
Cellulose is the most abundant natural polymer on earth, which serves as a main component comprised in a plant cell wall. Every year, several 100 billion tons of cellulose is photosynthesized. The cellulose is widely used as wood, paper pulp, fiber materials or cellulose derivatives due to very high crystallinity, molecular weight, and rigidity, and low solubility. Cellulosic plastics for industrial use are cellulose derivatives in which different materials bind to hydroxyl groups of cellulose to remove a hydrogen bond. The cellulose derivatives have different properties and biodegradabilities depending on the degree in substitution. Among the cellulose derivatives, cellulose diacetate (CDA) is the most widely used. CDA has been known as a source material for an acetate fiber and is used in the field of various applications as materials for the fiber industry, medical membranes, photographic films, and materials for cigarette filters. It has been known that CDA is prepared in a fiber or film type by solution spinning using acetone as a solvent, but a melting process has been recently introduced with the effective plasticization with a plasticizer. CDA is degraded at a processing temperature since it has a higher glass transition temperature (Tg) than general-purpose plastics. There fore, the Tg should be decreased under the degradation temperature to improve processability. However, plasticized CDA has increased processability, but tends to be decreased in mechanical properties and thermal stability.
The price of the biodegradable plastic is higher than that of conventional synthetic polymers, which is becoming the main issue. There is active research on preparation of composites in which a relatively cheap and abundant natural fiber is added to a biodegradable polymer in order to reduce the price of the biodegradable plastic and improve the mechanical properties. The natural fiber can be completely biodegradable in nature, and has a specific gravity of approximately 1.1 to 1.5, which accounts for about 50 to 60% of a glass fiber widely used in the current industry. Therefore, when such a natural fiber is applied to plastic, a lightweight and eco-friendly product can be obtained. Since the natural fiber absorbs a great deal of carbon dioxide from the air and emits oxygen during its cultivation and growth, it contributes to the prevention of global warming. Unlike the glass fiber or synthetic fiber, the natural fiber is not only reusable but also has no increased concentration of carbon dioxide exhausted to the air during incineration after being used. The natural fiber-engrafted composite is expected to play a more effective role as a reinforcer than the composites to which an inorganic mineral or metal reinforcer is engrafted. The natural fiber-engrafted composite is referred to as a green composite material or a bio composite material, and is known to be widely applied in materials for automobile parts, architecture and civil engineering, and consumer goods.
One of the natural fibers, kenaf, is a cellulosic natural fiber, which is usually produced in Southeast Asia, grows as fast as possible for a maximum of three crops a year, and is easily harvested and produced at a very low price. It has been known that plants on 1-hectare arable land can absorb about 30 to 40 tons of carbon dioxide in a cycle of cultivation.
However, since the natural fiber generally has a considerably low density compared to cellulose, it is difficult to mix and materials are not smoothly provided from a feeder during extrusion, which makes it impossible to increase a filling amount of the natural fiber.